Air pollution determination by mercury air sampling

ABSTRACT

Apparatus and system for detecting and sampling mercury vapor in the atmosphere, particularly for air pollution determination, utilizing sensitized absorption of the vapor on surfaces of noble metal wire grids. The wire grids operate to concrete encountered low levels of vapors. Release of mercury from the grids into a photometer for quantitation is achieved by direct passage of electrical current through the grid wire. The grids are designed to allow for ohmic heating of the absorbent wire to render possible a portable monitoring device. The apparatus and system incorporates means for volitilizing or decomposing mercury-bearing particulates for direct absorption of the vapor on the grids or in conjunction with a heated prefilter.

I Umted States Patent [151 3,640,624

Anderson et al. 1 Feb. 8, 1972 [54] AIR POLLUTION DETERMINATION BY 3,476,516 11/1969 Curry ..356/244 X MERCURY AIR SAMPLING 3,478,206 11/1969 Gaglione ...356/246 X 3,507,622 4/1970 Tammelln et al ..23/254 [72] Inventors: Howard H. Anderson, Covina; Rudolph ll.

Moyer, West Covina; Donald J. Sibbett, OTHER PUBLICATIONS cucamonga; David sllfllel'lfllld, l Krestovnikov,AnalyticalChem.,v. 65. 1966 p. 1374 Monte, all of Calif. 73 Assignee: Geomet, Incorporated, Rockville, Md. jggf f 'gfgfg ggg ggg [22] Filed: Apr. 29, 1970 AttorneyDavid H. Semmes [21] Appl. No.: 32,917 [57] ABSTRACT Apparatus and system for detecting and sampling mercury [52] US. Cl ..356/36, 73/23, 73/4215, vapor in the atmosphere particularly for air pollution dame 250/218 356/38 356/51 356/207 356/244 mination, utilizing sensitized absorption of the vapor on sur- [51] Int. Cl. ..G0ln 1/00, GOln 33/28,G01n21/12 I faces of noble metal wire grids The wire grids operae w [58] Field of Search ..350/36, 38, 206, 207, 208, concrete encountered low levels f vapors. Re|eaSe f merew 350/246, 244; 250/218; 73/231 4215 ry from the grids into a photometer for quantitation is achieved by direct passage of electrical current through the Refel'fllces cued grid wire. The grids are designed to allow for ohmic heating of UNITED STATES PATENTS tjzelizlgsorbent wire to render possible a portable monitoring 3,068,402 12/1962 Redhead ..324/33 The apparatus and system incorporates means for volitilizing 1,8811 16 10/1932 o r decomposing mercury-bearing particulates for direct ab- 2112311 8/1940 Pfund sorption of the vapor on the grids or in conjunction with a 2,263,335 11/1941 Heinz heated -m 3,027,552 3/1962 Landis ..356/201 X 8 Claims, 8 Drawing Figures PATENT-Ema 3 I972 sum 1 BF 3 BY [fl -124M ATTORNEY PAIENTEUFEB 8 I972 3.640.624

saw 2 or 3 ATTORNEY PATENTEBFEB a ma sum 3 or 3 ave /////4' aame/ wwz/w wwmww IA? ill Id)? HHHHHU /Z {I N 144/ pd/24M ATTORNEY AIR POLLUTION DETERMINATION BY MERCURY AIR SAMPLING BACKGROUND OF THE INVENTION Detection and measurement of the quantity of mercury found in air and soil for geological exploration has been used as an indicator during exploration for many base and precious metal ore deposits. The significance of mercury determinations in mineral explorations have been recognized as of substantial significance.

Utilization of the principle involved and appropriate instrumentation are deemed to have significant value upon realization of practical apparatus and systems to determine the amount of mercury found in the air for geological exploration, air pollution measurements, and laboratory toxicological monitoring. The first two applications are similar inasmuch as high sensitivity or the ability to measure various small mass concentrations is required. In these high sensitivity devices, the ability to detect about 1.0 nanogram per cubic meter of air, the approximate background level, establishes the instrument specifications. Levels as high as several hundred ngJm." are occasionally encountered, however.

While techniques and instrumentation have heretofore been devised primarily for geological exploration, the gas measuring instruments have utilized the following procedures:

1. Adsorption of the mercury from the air sampled onto the surface of a noble metal wettable by mercury;

2. Desorption as vapor by heating the collected mercury; and

3. Estimation of the mercury vapor by measurement of its optical adsorption at the wavelength of a mercury resonance line (253.7 nm., usually) in an ultraviolet photometer.

The prior devices as constituted were difficulty portable. They normally required that the samples of mercury collected from the air, the soil, or soil-air, be transported to a remote central analytical station for determination of the path-finder indications. Such a procedure is time consuming and inefficient with respect to directing geological surveys. In addition, in order to utilize a random arrangement of the adsorbent, usually gold or silver foil or wool, desorption was carried out with the assistance of an induction furnace. This is expensive, bulky, heavy and requires relatively high power inputs.

In addition to geological surveying, another, and now extremely important and urgent application of the principles involved, is use in measuring mercury-containing air pollutants. To date, substantially only standard chemical techniques have been widely employed, namely, collection by bubblers followed by wet chemical determinations of mercury. While some devices are known for monitoring laboratory air based on ultraviolet absorption of mercury, these are not sufficiently sensitive to levels found in the normal ambient atmospheres (0.5 to 25 ng./m. and are generally employed for laboratory safety monitoring.

A problem of some significance in monitoring air pollutants involves the need to measure the mercury content of particulates as well as vapors. These particulates arise from industrial processes as well as reactions between mercury vapor and gaseous and aerosolized contaminants. A major mercuric salt present as aerosolized particulates is believed to be mercuric chloride, a possible result of chemical equilibrium between aerosol components such as sea salt and mercury vapor. A necessity exists for methods for resolving problems associated with mercury vapor and aerosol monitors. With respect to air pollution determination and control apparatus and a system are urgently needed to permit measurement of the components whether particulate or in vapor form.

SUMMARY OF THE INVENTION The present invention while broadly pertaining to detecting and sampling mercury vapor in the atmosphere is more particularly adapted for use in conjunction with air pollution determination which involves the necessity to measure mercury content of particulates as well as vapors. The apparatus is readily portable and self-contained. Air is drawn in to the instrument which includes mercury vapor and mercury containing particulates and a heated filter-heat exchanger is incorporated at the air inlet to volitilize and/or decompose mercury bearing particulates, the resultant mercury vapor being adsorbed on a grid comprised of a set of noble metal, preferably silver, wire grids. The mercury containing vapors, after being heated, are adsorbed on two electrically separated grids utilizing a new technique of a consecutive heating method to delete interferences such as might be obtained from adsorbed hydrocarbons which would distort the obtained readings. Upon sequential grid section heating desorption of the collected mercury occurs and is carried by the air stream into a photometer. Resultant light absorption is measured by the photometer and, after appropriate calibration, directly relates the absolute amount of mercury vapor in the air including mercury contained in vapor and/or particulate form.

Additional features, techniques and advantages of the present invention will be more readily apparent from the following detailed description of an embodiment thereof when taken together with the accompanying drawings in which:

FIG. 1 is a perspective view, a portion being removed, of unified apparatus in accordance with the invention;

FIG. 2 is a plan view of the apparatus with the case top removed;

FIG. 3 is a sectional view of a mercury vapor collector as sembly taken substantially on line 3-3 of FIG. 2;

FIG. 4 is a side elevational view with the case side wall removed;

FIG. 5 is a front view external of the case incorporating the invention;

FIG. 6 is a rear view, in elevation, a portion of the case being removed;

FIG. 7 is a schematic of the electrical/electronics circuitry; and

FIG. 8 is a graph depicting photometer signals plotted against mercury vapor concentrations.

Referring to the drawings in detail there is shown a preferred design for practicing the invention utilizing the inventive concepts as indicated above. It is to be understood that the depicted apparatus is illustrative of a single preferred design and that other apparatus specifics can be utilized in practicing the concepts of the invention.

In the form of the invention shown the entire device is enclosed in a case 10 of a size for easy portability, such for example as 18% by 10-54; by 12 inches. As designed, the apparatus requires only ll0-120 volts, 60-cycle power to commence operation. Readout instrumentation which is not specifically shown can consist of an analog to digital converter and a printer for example.

Appropriately mounted within the casing, generally indicated, are a collector assembly 12, a blower unit 14, an absorption chamber 16, a removable housing containing a vacuum photodiode tube 18, a removable ultraviolet light housing assembly 20, an air inlet 22, an air outlet 24 and a chassis 26, preferably of aluminum, for electronics. A handle such as at 28 is provided on the case and an appropriate instrument controls area is provided at 30 on aluminum front panel 32. The case has appropriate legs 34 thereon.

The mercury vapor collector assembly 12 consists of an outer anodized aluminum tube 36 and a similar inner tube 38. An aluminum fairing 40 supports and plugs the front end of inner tube 38 which is also closed at its rear end by a removable plate 42, appropriate supports being otherwise provided and the inner and outer tubes being so dimensioned as to provide an annular duct 44 therebetween. The annular duct constitutes an air passage through the collector assembly. Ceramic rods 46 are inset and cemented into each corner of inner tube 38 with the outer surface having notches, not shown, 24 per inch to separate the grid wire, preferably silver, supported thereon. Preferably 0.020 diameter silver grid wire is wound on the rods on the inner tube at 24 turns per inch. Two consecutive windings 48A, 48B, FIG. 7, comprise the wire grid generally designated 48, the wire being shown at 50. Each consecutive winding is approximately 960 inches long and in the embodiment shown providing a total silver surface area of 120 inches squared. The construction is such that air flowing through the annular duct flows over the entire grid windings.

The entrance end for air at air inlet 33 includes a primary filter 52 detachably mounted as at 54 and inwardly thereof a heater grid assembly 56 and a cleanable metal filter 58 with appropriate spacers 60 and a stop ring 62. The construction provides for air intake from the ambient surrounding atmosphere upon actuation of blower unit 14 to impel preheated and prefiltered air through annular duct 44 over grid 48. The blower unit preferably uses a brushless 1 volt AC motor generally shown at 64 operatively associated with the body blower 66. The blower body is operatively interconnected at 68 into the rear end of outer tube 36 of the collector assembly. The blower body connects into air outlet 24.

A plenum chamber 70 interconnects the collector assembly with the blower inlet for introduction of air from the collector assembly through aluminum inlet tube 72 into absorption chamber 16. A baffle plate 74 having a large orifice 76 provides for airflow from the collector to the blower. A solenoid operated plate valve 78, shown in a collect position in FIG. 2 is mounted and operates in a manner when the plate closes the orifice 76 the inlet orifice to inlet tube 72 opens to the absorption chamber 16 and the blower draws air being sampled through chamber 16 as indicated by arrow 80 and thence through chamber outlet tube 82 as indicated by arrow 84 through discharge outlet 86 having a flow adjust valve 88 therein. The flow pattern of the air is indicated by arrows at 90 and controllable by function of the plate valve and flow adjust valve.

The absorption chamber 16 constitutes a portion of a photometer of a known type including a black anodized aluminum tubing 92 with welded end plates for attaching ultraviolet light housing assembly and photometer tube housing 18. The light housing assembly 20 includes a light filter 94, an ultraviolet light 96 and a concave reflector 98.

A schematic of the electrical and operating system is shown in FIG. 7. Sample air enters air inlet at 22 passing over heater 56 thence through collector assembly 12, passing over grid 48 with sections 48A, 48B being selectively actuated from a 110 volt AC power source 100 controllable by a timer 102 with the circuit including a fail-safe mechanism indicated at box 104. The timer 102 also controls the position of valve 78 for selected airflow control. Air then flows through tube 80 into absorption chamber 16 with the operatively associated photometer tube 106 and ultraviolet lamp 96. Exhaust airflows out of tube 84 in proper timed relationship for the sampling and testing phases. The photometer, specially designed for the desired airflow rates and geometry of the sampler grids and related heating requirements, utilizes a well-known technique. For example, a dual-beam type of photometer is described by Samuel H. Williston in Journal of Geophysical Research, Volume 73, Number 22 Nov. 15, 1968, pages 705 l7055 and a single beam device has been taught by W.W. Vaughn et al., in Geological Survey Research, 1964, U.S. Geological Survey, Prof. paper 50l-D pages Dl23-Dl27 and U.S. Geological Survey Circular 540, U.S. Department of Interior, 1957. Obviously different photometers can be utilized within the teachings of the invention and are well known to those skilled in the art. Additional details are not considered necessary herein.

The circuit operationally includes a bridge 108 from power source PS with operational amplifiers PP 55, SP2A, PP25, valves 110 and 112 and a low voltage power source LVPS. A set up and test box is indicated at 114. An analog to digital converter is schematically indicated at 1 16 and a result printer or readout at 118.

The mechanical operation is as follows:

Air, drawn in by the blower at 100 liters per minute passes the primary heated filter where particles containing mercury are trapped and heated to ISO-300 C. to desorb vapors or decompose or volitilize mercury compounds. Mercury particulates, such as mercuric chloride particles are readily volitilized or decomposed by this treatment. Mercuric chloride vapors are readily detectable.

After the initial heating step, the heated air plus vapors passes down the annulus between the inner and outer tubes across the silver wire grid windings. Mercury containing vapors are removed from the air during this step.

During the collection cycle, 5-60 minutes, (adjustable), the plate valve exhausts the air through the plenum chamber to the air outlet. During the processing cycle, the plate valve is in the closed position and passes air through the orifice controlling flow into the photometer cell. Simultaneously the first grid winding is heated, desorbing mercury and contaminants. These vapors flow across the second grid where the mercury vapors, being much more strongly adsorbed than the other vapor components, readsorb. The contaminants from the first grid are then measured in the photometer. This gives an electronic signal value V,. V, is the integrated area under the voltage-time curve obtained from the photometer when the first grid is heated.

After 20 seconds (adjustable), the second grid is heated. The optical absorption of these vapors in the photometer gives a value V V is the signal obtained from all the mercury vapor plus the desorbable contaminants which collect during the collection cycle and is the integrated area under the voltage-time curve obtained from the photometer when the second grid is heated. The difference V minus V, equals VS and is read as the system output. V is the difference between the two integrated voltage-time outputs. The signal difference V is directly proportional to the mercury concentration in the cell and can be read by a number of techniques depending upon the data handling technique which is selectable. The difference V is read as the system output which can be digital as indicated.

The schedule of the operation as controlled by the timer is as indicated in the following chart for a readout schedule requiring 1 minute:

READOUT CYCLE TIMING SCHEDULE In FIG. 8 the results of test data for the device indicates photometer signals (volts) plotted against mercury vapor concentrations (ng per M). It will be seen that the response of the device is linear over a range for the test conditions and existing circumstances.

While a preferred embodiment of apparatus has been shown and described and the principles of operation described with reference thereto, manifestly changes in minor details of construction and operation can be incorporated within the teachings of the invention as defined in and limited solely by the appended claims.

We claim:

1. In a system for sampling and measuring mercury content in ambient air, comprising:

A. a housing having an entrance end and an exit end through which air may flow;

B. means for blowing ambient air through said housing;

C. a grid collector positioned in said housing and including:

i. a first grid section;

ii. a second grid section;

iii. said first and second grid sections being spaced one behind the other in the direction of ambient airflow thereover; and

iv. said first and second grid sections each consisting of noble metal wire wound in convolute form for selectively adsorbing mercury vapor and contaminants from the ambient air;

D. means for selectively and sequentially heating said first and second grid sections to:

i. initially desorb mercury vapor and contaminants adsorbed on said first grid section for subsequent adsorption of mercury vapors on said second grid section; and

ii. subsequently desorb said mercury vapors adsorbed on said second grid section;

E. a photometer spaced from and subsequent to said grid sections in the direction of ambient airflow through said housing to:

i. measure contaminants in air desorbed from said first grid section to obtain a reference signal; and

ii. measure mercury vapor in air desorbed from said second grid section; and

F. comparison means to quantitatively compare the contaminants and mercury vapor measured respectively from said first and second grid sections indicative of mercury vapor in a quantity of sampled ambient air.

2. in a system as claimed in claim 1, means to decompose and volitilize mercury-containing particulates prior to collection on said first grid section.

3. In a system as claimed in claim 2, said means to decompose and volitilize mercury-containing particulates comprising a heated prefilter.

4. In a system as claimed in claim 1, said housing comprising a collector assembly consisting of spaced inner and outer tubes defining an annular duct therebetween, said annular duct constituting an air passage through the assembly, said wire grid sections comprising consecutive wire windings insulatedly wound on said inner tube and positioned in said annular duct for passage thereover of the ambient air.

5. In a system as claimed in claim 4, an absorption chamber including said photometer and an ultraviolet light, mercury, and contaminants in the ambient air being quantitatively measured by light absorption in the absorption chamber by said photometer.

6. In a system as claimed in claim 5, the collector assembly and absorption chamber being interconnected for sequential airflow and passage therethrough, and valve means for selective timed sequence of flow of air from the collector assembly into the absorption chamber and exit from the collector and absorption assemblies.

7. A method for sampling and measuring mercury content in ambient air for air pollution determination, comprising:

A. sampling quantities of ambient air and collecting mercury in vapor or mercury-containing particulate form contained in the ambient air by blowing ambient air across a noble metal wire grid collector consisting of a first grid section and a second grid section spaced in the direction of airflow for adsorbing mercury vapor and contaminants from the ambient air on the first grid section;

B. selectively and sequentially heating said first and second grid sections to initially desorb collected mercury and contaminants from said first grid section, readsorb mercury vapor on said second grid section, and subsequently desorb mercury vapor collected on said second grid section;

C. passing ambient air and contaminants as desorbed from said first grid section through a photometer and measuring contaminants and obtaining a reference signal indicative of the contaminants;

D. passing ambient air and mercury vapor as desorbed from said second grid section through the photometer and measuring the mercury vapor in the air; and

E. comparing the amount of contaminants and the amount of mercury vapor contents as measured in the photometer in a comparison means to quantitatively determine the amount of mercury vapor in a quantity of sampled ambient air. 8. A method as claimed in claim 7, and including the step of preheating and filtering ambient air prior to mercury collection to decompose and volitilize mercury-containing particulates in air being sampled. 

2. In a system as claimed in claim 1, means to decompose and volitilize mercury-containing particulates prior to collection on said first grid section.
 3. In a system as claimed in claim 2, said means to decompose and volitilize mercury-containing particulates comprising a heated prefilter.
 4. In a system as claimed in claim 1, said housing comprising a collector assembly consisting of spaced inner and outer tubes defining an annular duct therebetween, said annular duct constituting an air passage through the assembly, said wire grid sections comprising consecutive wire windings insulatedly wound on said inner tube and positioned in said annular duct for passage thereover of the ambient air.
 5. In a system as claimed in claim 4, an absorption chamBer including said photometer and an ultraviolet light, mercury, and contaminants in the ambient air being quantitatively measured by light absorption in the absorption chamber by said photometer.
 6. In a system as claimed in claim 5, the collector assembly and absorption chamber being interconnected for sequential airflow and passage therethrough, and valve means for selective timed sequence of flow of air from the collector assembly into the absorption chamber and exit from the collector and absorption assemblies.
 7. A method for sampling and measuring mercury content in ambient air for air pollution determination, comprising: A. sampling quantities of ambient air and collecting mercury in vapor or mercury-containing particulate form contained in the ambient air by blowing ambient air across a noble metal wire grid collector consisting of a first grid section and a second grid section spaced in the direction of airflow for adsorbing mercury vapor and contaminants from the ambient air on the first grid section; B. selectively and sequentially heating said first and second grid sections to initially desorb collected mercury and contaminants from said first grid section, readsorb mercury vapor on said second grid section, and subsequently desorb mercury vapor collected on said second grid section; C. passing ambient air and contaminants as desorbed from said first grid section through a photometer and measuring contaminants and obtaining a reference signal indicative of the contaminants; D. passing ambient air and mercury vapor as desorbed from said second grid section through the photometer and measuring the mercury vapor in the air; and E. comparing the amount of contaminants and the amount of mercury vapor contents as measured in the photometer in a comparison means to quantitatively determine the amount of mercury vapor in a quantity of sampled ambient air.
 8. A method as claimed in claim 7, and including the step of preheating and filtering ambient air prior to mercury collection to decompose and volitilize mercury-containing particulates in air being sampled. 